This invention is directed to control systems, and, more particularly, to closed loop control systems for controlling the operation of a stepping motor.
The present invention was developed for use in combination with a serial printer and is described in such an environment. However, as will be appreciated by those skilled in the stepping motor art, and others, the invention is useful in other environments wherein stepping motors are utilized to move, or control the position of a mechanical device or devices.
As used herein the term serial printer refers to a printer wherein a print head is skewed or moved, either uni-directionally or bi-directionally across a print receiving medium (paper), during a printing operation. The print head may cause an integral character to be formed each time the head is moved into contact with the print receiving medium (either directly or via a print ribbon), or only a portion of a character, as occurs in a dot matrix type serial printer.
One of the problems relating to the use of stepping motors to move a mechanical object, such as a serial printer print head, is maintaining the velocity of movement of the object precisely constant. That is, as will be readily understood by those skilled in the stepping motor art, the application of a series of motor step pulses to a stepping motor causes the motor's shaft to accelerate to a velocity related to the frequency of the pulses. The motor shaft continues to rotate at a constant average velocity as long as the frequency of the motor step pulses remains constant. If the frequency varies, the velocity of movement of the motor shaft will vary in a corresponding manner. The problem occurs because, while the average velocity is maintained constant, the instantaneous velocity will vary slightly and overshoot and undershoot the average velocity due to a variety of factors, the most important of which is load fluctuation, i.e., the load applied to the object being moved. In the case of a print head load fluctuation is caused by the printing operation. Other factors such as friction, motor characteristics, etc. also have an effect on the instantaneous velocity of the stepping motor shaft, even though the average velocity remains constant. Obviously, if the instantaneous velocity is not constant or substantially constant related functions are not exactly repeatable and may not occur at the precise point that they should occur. For example, in order for a dot matrix serial printer to produce images as clear as desired, each dot column must be printed at precise point. If not, the image lacks clarity to the extent one or more dot columns is skewed, i.e., misplaced.
The foregoing difficulty becomes particularly acute when the serial printer prints bi-directionally as opposed to unidirectionally. In such an environment, a character may be skewed in one direction in one line and another, adjacent, character skewed in the other direction in the next line, resulting in an unacceptable print.
Therefore, it is an object of this invention to provide a new and improved stepping motor control system.
It is a further object of this invention to provide a closed loop stepping motor control system for controlling the rotational velocity of a stepping motor such that the instantaneous velocity remains substantially constant.
It is another object of this invention to provide a stepping motor control system having substantially less over and under shoot about an average velocity than a system wherein a constant frequency source is used to control the operation of a stepping motor.
It is a still further object of this invention to provide a closed loop control system for controlling the rotational velocity of a stepping motor in either direction during periods of continuous shaft rotation.